if localStorage key value doesn't exist - html

I am trying to hide my div if there is no a localStorage key value.
With the line below I achieved only to hide a div when localStorage key completely deleted but need to do the same if localStorage key hasn't got any value at all just [].
window.localStorage.getItem('items') === null
How would it be performed?

You can add required conditions using the OR operator ||
var items = window.localStorage.getItem('items')
if (items === null || items.length === 0)
{
// items is null, [] or '' (empty string)
}
If you have to check for undefined somewhere as well you can change === null to == null or expand with an extra condition like this
if (items === undefined || items === null || items.length === 0)
{
// items is undefined, null, [] or '' (empty string)
}
EDIT: Here is what you can do to get the array directly
var items = JSON.parse(window.localStorage.getItem('items'))
if (items === null || items.length === 0)
{
// items is null or []
}

How about simply:
if (!localStorage.nameOfYourLocalStorage) {
// do the following if the localStorage.nameOfYourLocalStorage does not exist
}
An example of how it could be useful:
if (!localStorage.nameOfYourLocalStorage) {
localStorage.nameOfYourLocalStorage = defaultValue;
}
Here the script will check if the localStorage name does not exist, and if it doesn't, it will create it with the default value.
And if you want it to act when it does exist, you can add an else after the if block to continue, or remove the '!' from the if block condition.

Storage.prototype.has = function(key) {
return (key in this && !(key in Storage.prototype));
};

Related

javascript backtick and jquery not working [duplicate]

How do I determine if variable is undefined or null?
My code is as follows:
var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName == 'undefined'){
// DO SOMETHING
};
<div id="esd-names">
<div id="name"></div>
</div>
But if I do this, the JavaScript interpreter halts execution.
You can use the qualities of the abstract equality operator to do this:
if (variable == null){
// your code here.
}
Because null == undefined is true, the above code will catch both null and undefined.
The standard way to catch null and undefined simultaneously is this:
if (variable == null) {
// do something
}
--which is 100% equivalent to the more explicit but less concise:
if (variable === undefined || variable === null) {
// do something
}
When writing professional JS, it's taken for granted that type equality and the behavior of == vs === is understood. Therefore we use == and only compare to null.
Edit again
The comments suggesting the use of typeof are simply wrong. Yes, my solution above will cause a ReferenceError if the variable doesn't exist. This is a good thing. This ReferenceError is desirable: it will help you find your mistakes and fix them before you ship your code, just like compiler errors would in other languages. Use try/catch if you are working with input you don't have control over.
You should not have any references to undeclared variables in your code.
Combining the above answers, it seems the most complete answer would be:
if( typeof variable === 'undefined' || variable === null ){
// Do stuff
}
This should work for any variable that is either undeclared or declared and explicitly set to null or undefined. The boolean expression should evaluate to false for any declared variable that has an actual non-null value.
if (variable == null) {
// Do stuff, will only match null or undefined, this won't match false
}
if (typeof EmpName != 'undefined' && EmpName) {
will evaluate to true if value is not:
null
undefined
NaN
empty string ("")
0
false
Probably the shortest way to do this is:
if(EmpName == null) { /* DO SOMETHING */ };
Here is proof:
function check(EmpName) {
if(EmpName == null) { return true; };
return false;
}
var log = (t,a) => console.log(`${t} -> ${check(a)}`);
log('null', null);
log('undefined', undefined);
log('NaN', NaN);
log('""', "");
log('{}', {});
log('[]', []);
log('[1]', [1]);
log('[0]', [0]);
log('[[]]', [[]]);
log('true', true);
log('false', false);
log('"true"', "true");
log('"false"', "false");
log('Infinity', Infinity);
log('-Infinity', -Infinity);
log('1', 1);
log('0', 0);
log('-1', -1);
log('"1"', "1");
log('"0"', "0");
log('"-1"', "-1");
// "void 0" case
console.log('---\n"true" is:', true);
console.log('"void 0" is:', void 0);
log(void 0,void 0); // "void 0" is "undefined"
And here are more details about == (source here)
BONUS: reason why === is more clear than == (look on agc answer)
jQuery attr() function returns either a blank string or the actual value (and never null or undefined). The only time it returns undefined is when your selector didn't return any element.
So you may want to test against a blank string. Alternatively, since blank strings, null and undefined are false-y, you can just do this:
if (!EmpName) { //do something }
Edited answer: In my opinion, you shouldn't use the function from my below old answer. Instead, you should probably know the type of your variable and use the according to check directly (for example, wondering if an array is empty? just do if(arr.length===0){} etc.). This answer doesn't even answer OP's question.
I've come to write my own function for this. JavaScript is weird.
It is usable on literally anything. (Note that this also checks if the variable contains any usable values. But since this information is usually also needed, I think it's worth posting). Please consider leaving a note.
function empty(v) {
let type = typeof v;
if (type === 'undefined') {
return true;
}
if (type === 'boolean') {
return !v;
}
if (v === null) {
return true;
}
if (v === undefined) {
return true;
}
if (v instanceof Array) {
if (v.length < 1) {
return true;
}
} else if (type === 'string') {
if (v.length < 1) {
return true;
}
if (v === '0') {
return true;
}
} else if (type === 'object') {
if (Object.keys(v).length < 1) {
return true;
}
} else if (type === 'number') {
if (v === 0) {
return true;
}
}
return false;
}
TypeScript-compatible.
This function should do exactly the same thing like PHP's empty() function (see RETURN VALUES)
Considers undefined, null, false, 0, 0.0, "0" {}, [] as empty.
"0.0", NaN, " ", true are considered non-empty.
The shortest and easiest:
if(!EmpName ){
// DO SOMETHING
}
this will evaluate true if EmpName is:
null
undefined
NaN
empty
string ("")
0
false
If the variable you want to check is a global, do
if (window.yourVarName) {
// Your code here
}
This way to check will not throw an error even if the yourVarName variable doesn't exist.
Example: I want to know if my browser supports History API
if (window.history) {
history.back();
}
How this works:
window is an object which holds all global variables as its properties, and in JavaScript it is legal to try to access a non-existing object property. If history doesn't exist then window.history returns undefined. undefined is falsey, so code in an if(undefined){} block won't run.
In JavaScript, as per my knowledge, we can check an undefined, null or empty variable like below.
if (variable === undefined){
}
if (variable === null){
}
if (variable === ''){
}
Check all conditions:
if(variable === undefined || variable === null || variable === ''){
}
Since you are using jQuery, you can determine whether a variable is undefined or its value is null by using a single function.
var s; // undefined
jQuery.isEmptyObject(s); // will return true;
s = null; // defined as null
jQuery.isEmptyObject(s); // will return true;
// usage
if(jQuery.isEmptyObject(s)){
alert('Either variable: s is undefined or its value is null');
}else{
alert('variable: s has value ' + s);
}
s = 'something'; // defined with some value
jQuery.isEmptyObject(s); // will return false;
I've just had this problem i.e. checking if an object is null.
I simply use this:
if (object) {
// Your code
}
For example:
if (document.getElementById("enterJob")) {
document.getElementById("enterJob").className += ' current';
}
You can simply use the following (I know there are shorter ways to do this, but this may make it easier to visually observe, at least for others looking at the code).
if (x === null || x === undefined) {
// Add your response code here, etc.
}
source: https://www.growthsnippets.com/how-can-i-determine-if-a-variable-is-undefined-or-null/
jQuery check element not null:
var dvElement = $('#dvElement');
if (dvElement.length > 0) {
// Do something
}
else{
// Else do something else
}
With the newest javascript changes, you can use the new logical operator ??= to check if the left operand is null or undefined and if so assign the value of right operand.
SO,
if(EmpName == null){ // if Variable EmpName null or undefined
EmpName = 'some value';
};
Is equivalent to:
EmpName ??= 'some value';
The easiest way to check is:
if(!variable) {
// If the variable is null or undefined then execution of code will enter here.
}
I run this test in the Chrome console. Using (void 0) you can check undefined:
var c;
undefined
if (c === void 0) alert();
// output = undefined
var c = 1;
// output = undefined
if (c === void 0) alert();
// output = undefined
// check c value c
// output = 1
if (c === void 0) alert();
// output = undefined
c = undefined;
// output = undefined
if (c === void 0) alert();
// output = undefined
With the solution below:
const getType = (val) => typeof val === 'undefined' || !val ? null : typeof val;
const isDeepEqual = (a, b) => getType(a) === getType(b);
console.log(isDeepEqual(1, 1)); // true
console.log(isDeepEqual(null, null)); // true
console.log(isDeepEqual([], [])); // true
console.log(isDeepEqual(1, "1")); // false
etc...
I'm able to check for the following:
null
undefined
NaN
empty
string ("")
0
false
To test if a variable is null or undefined I use the below code.
if(typeof sVal === 'undefined' || sVal === null || sVal === ''){
console.log('variable is undefined or null');
}
if you create a function to check it:
export function isEmpty (v) {
if (typeof v === "undefined") {
return true;
}
if (v === null) {
return true;
}
if (typeof v === "object" && Object.keys(v).length === 0) {
return true;
}
if (Array.isArray(v) && v.length === 0) {
return true;
}
if (typeof v === "string" && v.trim().length === 0) {
return true;
}
return false;
}
(null == undefined) // true
(null === undefined) // false
Because === checks for both the type and value. Type of both are different but value is the same.
Let's look at this,
let apple; // Only declare the variable as apple
alert(apple); // undefined
In the above, the variable is only declared as apple. In this case, if we call method alert it will display undefined.
let apple = null; /* Declare the variable as apple and initialized but the value is null */
alert(apple); // null
In the second one it displays null, because variable of apple value is null.
So you can check whether a value is undefined or null.
if(apple !== undefined || apple !== null) {
// Can use variable without any error
}
The foo == null check should do the trick and resolve the "undefined OR null" case in the shortest manner. (Not considering "foo is not declared" case.) But people who are used to have 3 equals (as the best practice) might not accept it. Just look at eqeqeq or triple-equals rules in eslint and tslint...
The explicit approach, when we are checking if a variable is undefined or null separately, should be applied in this case, and my contribution to the topic (27 non-negative answers for now!) is to use void 0 as both short and safe way to perform check for undefined.
Using foo === undefined is not safe because undefined is not a reserved word and can be shadowed (MDN). Using typeof === 'undefined' check is safe, but if we are not going to care about foo-is-undeclared case the following approach can be used:
if (foo === void 0 || foo === null) { ... }
You can do something like this, I think its more efficient for multiple value check on the same variable in one condition
const x = undefined;
const y = null;
const z = 'test';
if ([undefined, null].includes(x)) {
// Will return true
}
if ([undefined, null].includes(y)) {
// Will return true
}
if ([undefined, null].includes(z)) {
// Will return false
}
No one seems to have to posted this yet, so here we go:
a?.valueOf() === undefined works reliably for either null or undefined.
The following works pretty much like a == null or a == undefined, but it could be more attractive for purists who don't like == 😎
function check(a) {
const value = a?.valueOf();
if (value === undefined) {
console.log("a is null or undefined");
}
else {
console.log(value);
}
}
check(null);
check(undefined);
check(0);
check("");
check({});
check([]);
On a side note, a?.constructor works too:
function check(a) {
if (a?.constructor === undefined) {
console.log("a is null or undefined");
}
}
check(null);
check(undefined);
check(0);
check("");
check({});
check([]);
Calling typeof null returns a value of “object”, as the special value null is considered to be an empty object reference. Safari through version 5 and Chrome through version 7 have a quirk where calling typeof on a regular expression returns “function” while all other browsers return “object”.
var x;
if (x === undefined) {
alert ("only declared, but not defined.")
};
if (typeof y === "undefined") {
alert ("not even declared.")
};
You can only use second one: as it will check for both definition and declaration
var i;
if (i === null || typeof i === 'undefined') {
console.log(i, 'i is undefined or null')
}
else {
console.log(i, 'i has some value')
}
I still think the best/safe way to test these two conditions is to cast the value to a string:
var EmpName = $("div#esd-names div#name").attr('class');
// Undefined check
if (Object.prototype.toString.call(EmpName) === '[object Undefined]'){
// Do something with your code
}
// Nullcheck
if (Object.prototype.toString.call(EmpName) === '[object Null]'){
// Do something with your code
}

How to remove specific attribute in mxcell

I have a user object for which i have set attribute.
for setting the attribute i used
cell.setAttribute("firstName", "john")
but now i want to delete the attribute or remove it.
But there is no function to remove the attribute.
can anyone please help me to delete the attribute.
Looking at the implementation of setAttribute you can find:
mxCell.prototype.setAttribute = function(name, value)
{
var userObject = this.getValue();
if (userObject != null &&
userObject.nodeType == mxConstants.NODETYPE_ELEMENT)
{
userObject.setAttribute(name, value);
}
};
My suggestion would be to do the same: get the userObject with getValue first and then, considering it a regular node, use the removeAttribute function.
EDIT:
Further elaborating... You can implement your own "removeAttribute" function:
mxCell.prototype.removeAttribute = function(name)
{
var userObject = this.getValue();
if (userObject != null &&
userObject.nodeType == mxConstants.NODETYPE_ELEMENT)
{
userObject.removeAttribute(name);
}
};
First getIndex of:
mxCell.prototype.getIndex = function(name)
Then delete using index:
mxCell.prototype.remove = function( index )

How to filter JSON data using Typescript?

I have json data like this
jsonList= [
{name:'chennai', code:'maa'}
{name:'delhi', code:'del'}
....
....
....
{name:'salem', code:'che'}
{name:'bengaluru',code:'blr'}
}]
now i need to filter this data based on keys(name, code)and return matching values. For example if CHE is given, first i need to check on CODE if no values match then i need to check on NAME
{name:'salem', id:'che'},
{name:'chennai', id:'maa'}
I tried below code, but its not working. Each time it checks only on NAME
public filterJson(text){
this.filteredOptions = this.jsonList.filter(
e => (
e.code.toLowerCase().indexOf(text.toString().toLowerCase()) !== -1) ||
e.name.toLowerCase().indexOf(text.toString().toLowerCase()) !== -1)
).slice(0, 9);
}
and tried with 0
public filterJson(text){
this.filteredOptions = this.jsonList.filter(
e => (
e.code.toLowerCase().indexOf(text.toString().toLowerCase()) === 0) ||
e.name.toLowerCase().indexOf(text.toString().toLowerCase()) === 0)
).slice(0, 9);
}
type script playground link
The following works for me (fixing several syntax errors, one additional variable, no logic change)
var jsonList= [
{name:'chennai', code:'maa'}, // added commas
{name:'delhi', code:'del'},
{name:'salem', code:'che'},
{name:'bengaluru',code:'blr'},
/* removed redundant closing brace */ ];
function filterJson(text) {
const lcText = text.toString().toLowerCase(); // calculate this once
return jsonList.filter(
e => (
// Added initial opening brace
(e.code.toLowerCase().indexOf(lcText) === 0) ||
(e.name.toLowerCase().indexOf(lcText) === 0)
)// added closing brace
).slice(0, 9);
}
JSON.stringify(filterJson("che"))
"[{"name":"chennai","code":"maa"},{"name":"salem","code":"che"}]"
Are you sure your code compiled? This is one of the main benefits of TypeScript, it checks your syntax at compile time.
Answer already given by #Motti :) just want to update how to do sort, it might be helpful someone.
this.jsonList.filter(
e => (
(e.code.toLowerCase().indexOf(text) === 0) ||
(e.name.toLowerCase().indexOf(text) === 0)
)).sort(
(a, b) =>
(
(a.code.toLowerCase().indexOf(text) === 0) ? -1 :
(b.code.toLowerCase().indexOf(text) === 0) ? 1 : 0
)
)
.slice(0, 10);

HTML5 - type="number" prevents non-numbers in Chrome but not in IE & FF

I'm coding a site in MVC and I have a price field in my form.
My Razor code looks like this:
#Html.EditorFor(model => model.Price, new { htmlAttributes = new { #class = "myclass", min = 0, max = 1000000 } })
and my ViewModel property looks like this:
[Display(Name = "Price")]
[Range(0, 1000000, ErrorMessage="Please enter a valid Price")]
public int Price { get; set; }
In Chrome the user can't enter any other characters than . but in Firefox and Internet Explorer they can, which is not what I want.
Is it correct that HTML5 standards do not extend to the UI? And does Chrome doing a bonus? Or should IE and FF block letters (e.g. characters like $) and I have done something wrong?
EDIT: If I begin the input with, say, $ in IE it blanks the control upon losing focus. In Firefox it remains.
I resolved this by using JQuery. These are the steps:
1) Create an external file to load - MyUtils.js
2) Paste in this JQuery code:
// Use like: $('#SomeId').forceNumeric()
jQuery.fn.forceNumeric = function (intsOnly) {
return this.each(function () {
$(this).keydown(function (e) {
var key = e.which || e.keyCode;
if (!e.shiftKey && !e.altKey && !e.ctrlKey &&
// numbers
key >= 48 && key <= 57 ||
// Numeric keypad
key >= 96 && key <= 105 ||
// Backspace and Tab and Enter
key == 8 || key == 9 || key == 13 ||
// Home and End
key == 35 || key == 36 ||
// left and right arrows
key == 37 || key == 39 ||
// Del and Ins
key == 46 || key == 45)
{
// ints only: look for a decimal. Disallow comma
// minus and period on keypad
if ((intsOnly) && (key == 190 || key == 188 || key == 109 || key == 110)) {
return false;
}
// Input is OKAY
return true;
}
return false;
});
});
}
3) On the page you want to use this, load the js file
e.g. script src="/Scripts/MyUtils.js"
4) Apply the method to the field on setup
$('#Price').forceNumeric(true);

i want to allowed either null or integer value throw my jsp

var char1 = /^[0-9]+$/;
if (!($("#AcNo").val().match(char1))) {
window.parent.showAlert('<s:text name="common.lbl.AcNo" /> <s:text name="common.msg.notValid" />');
return false;
}
The value of a field can not be null, it's always a string value.
You should check for empty string instead.
if ( $("#AcNo").val() != '' ){
// your code here.
}
OR
if ( $("#AcNo").val().length != 0){
// your code here.
}
To check for numeric values (that includes floats and integers )
$.isNumeric($("#AcNo").val())
As per your comment. The following code checks if the value is not null and is an integer.
var value = $("#AcNo").val()
if( value != '' && value % 1 === 0 ) {
//do something
}